Method of producing oxide dielectric element, and memory and semiconductor device using the element

ABSTRACT

The temperature at which an oxide dielectric thin film is formed can be made lower than conventional by reducing the concentration of oxygen in an atmosphere for forming the thin film. As a result, there can be formed an oxide dielectric thin film which has a crystal structure preferentially oriented at a crystal plane allowing a polarization axis to be directed in the vertical direction, which eliminates any reaction with an electrode material, and controls the growth of crystal grains. The use of such an oxide dielectric thin film can provide an oxide dielectric element having a high spontaneous polarization and a small coercive field. Consequently, it is possible to achieve a dielectric element having a high density of integration for detecting reading and writing operations, and a semiconductor device using the same.

TECHNICAL FIELD

The present invention relates to a method of producing an oxidedielectric element, and a memory and a semiconductor device using theelement.

BACKGROUND OF THE INVENTION

One type of recent semiconductor memory is a ROM (Read Only Memory)which makes use of a non-volatile characteristic allowing retention ofdata even in the OFF-state of the power supply to the memory. The ROM,however, has problems in that the number of re-writing operations islargely limited, the re-writing speed is low, and the like. Another typeof recent semiconductor memory is a RAM (Random Access Memory) havingthe advantage of enabling re-writing of data at a high speed. An oxidedielectric substance is used as a material of a storing capacitor, whichis a basic component of the RAM. Oxide dielectric substances can beclassified into a high dielectric substance having a high dielectricconstant and a ferroelectric substance having a hysteresis ofpolarization. In particular, a DRAM using a high dielectric substanceand a non-volatile RAM using a ferroelectric substance are known. First,the non-volatile PAM using a ferroelectric substance will be described.Such a non-volatile RAM is advantageous in that a non-volatilecharacteristic is obtained by making use of the hysteresis effect of theferroelectric substance; the number of re-writing operations possible isas large as 10¹⁰ to 10¹²; and the re-writing speed is in μs ({fraction(1/1,000,000)} sec) or faster in comparison with other types of memory,and therefore, the non-volatile RAM is expected to become the futureideal memory. Developments have been made to further enhance thecapacity, the non-volatile characteristic and the re-writing speed ofthe non-volatile RAM. The non-volatile RAM, however, has a large problemin that film fatigue occurs with an increase in the number of re-writingoperations, whereby the spontaneous polarization (Pr) property of theferroelectric substance deteriorates. To enhance the capacity anddurability of the non-volatile RAM, it is known to adopt (1) aferroelectric material having a large spontaneous polarization (Pr) and(2) a ferroelectric material which is fatigue-free. As a ferroelectricmaterial having a large spontaneous polarization (Pr) and which isfatigue-free, an oxide having a perovskite structure is extensivelyavailable. In particular, there is a Bi-layered ferroelectric substance,SrBi₂Ta₂O₉, having a crystal structure in which a plurality of simplelattices of the perovskite structure are layered. This material has acrystal anisotropy allowing the spontaneous polarization Pr to bedirected only in a direction perpendicular to the c-axis, and thematerial has a good film fatigue characteristic, although the Pr valueis not necessarily large. Examples of devices using such a material aredisclosed in WO93/12542 (PCT/US92/10627) and Japanese Patent Laid-openNo. Hei 5-24994.

Meanwhile, a DRAM using a high dielectric substance has ridden on thewave of development requiring a large capacity, for example, 16 Mbits,or even 64 Mbits, along with the advancements in high density and highintegration techniques. To meet such a requirement, there has been astrong demand for making the geometries of a circuit component of theDRAM finer, particularly with respect to the geometries of a capacitorfor storing information. To achieve the finer-geometries of thecapacitor, attempts have been made to make a film of a dielectricmaterial thin, to adopt a material having a high dielectric constant,and to assemble the structure having top and bottom electrodes and adielectric substance not in two dimensions, but in three dimensions. Forexample, a high dielectric substance, BST [(Ba/Sr)TiO₃], having acrystal structure composed of a simple lattice of the perovskitestructure is a material having a dielectric constant (∈) larger thanthat of a conventional high dielectric substance, SiO₂/Si₃N₄. An exampleusing such a high dielectric substance has been reported in IEDM(International Electron Device Meeting) Tech. Dig.: 823, 1991.

SUMMARY OF THE INVENTION

The present invention relates to a method of producing an oxidedielectric element, and a memory and a semiconductor device using thesame. In particular, the present invention is applicable to highdielectric elements, such as a DRAM, which makes use of a highdielectric constant and a low leakage current density, and anon-volatile RAM, which makes use of a high spontaneous polarization anda low coercive field; and a memory and a semiconductor device using thehigh dielectric element or the ferroelectric element.

Ferroelectric thin films and high dielectric thin films have beenrequired to be heated up to high temperatures, for example, about 650°C. for a thin film of Pb(Zr/Ti)O₃, about 600° C. for a thin film of(Ba/Sr)TiO₃, and about 800° C. for a thin film of SrBi₂Ta₂O₉. That is tosay, in the formation of a thin film of the above material having aperovskite type crystal structure, the material must be heated up to atemperature of 600° C. or higher for promoting crystallization. The heattreatment at such a high temperature, however, causes various problems.For example, in the formation of a film by a vapor deposition process, abottom electrode is exposed to an oxidizing atmosphere at a hightemperature at the initial stage of the film formation, and thus issusceptible of being peeled off. Furthermore, when a film of SrBi₂Ta₂O₉is formed at a high temperature of 800° C. as conventional, Bi isevaporated to cause a deviation of the film composition, so that thestarting content of Bi is required to be excessive. As a result, afterthe film formation at a high temperature, the excessive Bi exists as anirregular phase containing Bi in a large amount at grain boundaries ofthe ferroelectric layer, which causes degradation of the withstandvoltage characteristic, and further, a transient layer is formed bydiffusion of elements at the interplane between the ferroelectric thinfilm and each of the top and bottom electrodes, which reduces thespontaneous polarization (Pr) to thereby degrade the originalcharacteristics of the ferroelectric element, increases the coercivefield (Ec), and causes the film fatigue. For this reason, the number ofre-writing operations performed by reversing an electric field islargely limited. Further, the heat treatment at a high temperaturecreates problems such as (a) the formation of the reaction layer reducesthe dielectric constant and spontaneous polarization, and (b) the growthof crystal grain increases the leakage current density. This leads to anincreased operational voltage, thus making it difficult to achieve highintegration of the element.

The present invention has been made on the basis of the above knowledge,and an object of the present invention is to provide a method ofproducing an oxide dielectric element having good characteristics,particularly, a ferroelectric element having a high spontaneouspolarization and a low coercive field, or a high dielectric elementhaving a high dielectric constant and a good withstand voltagecharacteristic; and to provide a memory and a semiconductor device usingthe above oxide dielectric element.

According to one feature of the present invention, there is provided amethod of producing an oxide dielectric element, particularly, aferroelectric element having a high spontaneous polarization and a lowcoercive field or a high dielectric element having a high dielectricconstant and a good withstand voltage characteristic, characterized inthat a ferroelectric thin film as a main part of the ferroelectricelement or a high dielectric thin film as a main part of the highdielectric element is formed in a low oxygen concentration atmosphere ata temperature of 650° C. or less for the ferroelectric thin film or 600°C. or less for the high dielectric thin film. In this case, to maximizethe occupied ratio of a perovskite structure in the entire crystal phasein the thin film and hence to enhance the electric characteristics ofthe element, the concentration of oxygen in the low oxygen concentrationatmosphere may be preferably set in a range which is greater than 0.1%and less than 5.0%.

According to another feature of the present invention, there is provideda method of producing an oxide dielectric element, characterized in thatthe low oxygen concentration atmosphere is made variable by adjustingthe mixing ratio of oxygen to inert gas, and the heat treatment iscarried out at atmospheric pressure. With this configuration, theproduction method can be made very simple.

According to a further feature of the present invention, there isprovided a method of producing an oxide dielectric element,characterized in that the ferroelectric thin film or high dielectricthin film formed in accordance with the above-described productionmethod is re-heated in an activated oxygen atmosphere of O₃, N₂O,radical oxygen or the like. With this configuration, the quality of theferroelectric or high dielectric thin film can be enhanced.

According to the present invention, the ferroelectric thin film ischaracterized in that it is expressed by a chemical structural formulaof (AO)²⁺(BCO)²⁻ where A is one kind of element selected from a groupconsisting of Bi, Tl, Hg, Pb, Sb and As; B is at least one kind ofelement selected from a group consisting of Pb, Ca, Sr, Ba and rareearth elements; and C is at least one kind of element selected from agroup consisting of Ti, Nb, Ta, W, Mo, Fe, Co and Cr; or expressed by achemical structural formula of (Pb/A) (Zr/Ti)O₃ where A is one kind ofelement selected from a group consisting of La, Ba and Nb.

The high dielectric thin film is characterized in that it is expressedby a chemical structural formula of (Ba/Sr)TiO₃.

The high dielectric thin film obtained according to the presentinvention is characterized in that it has a dielectric constant largerthan that of Ta₂O₅ conventionally used.

Each of the top and bottom electrode materials used in accordance withthe present invention is characterized in that it consists of at leastone kind of metal selected from a group consisting of Pt, Au, Al, Ni,Cr, Ti, Mo and W; at least one kind of conductive oxide of a singleelement selected from a group consisting of Ti, V, Eu, Cr, Mo, W, Ph,Os, Ir, Pt, Re, Ru and Sn; or at least one kind of conductive oxidehaving a perovskite structure selected from a group consisting of ReO₃,SrReO₃, BaReO₃, LaTiO₃, SrVO₃, CaCrO₃, SrCrO₃, SrFeO₃,La_(1−x)Sr_(x)CoO₃ (0<x<0.5), LaNiO₃, CaRuO₃, SrRuO₃, SrTiO₃ and BaPbO₃.In the case of using a conductive oxide of a single element or aconductive oxide having a perovskite structure, the oxide ischaracterized in that it has a resistivity of 1 mΩ·cm or less forensuring the function of an electrode material.

According to the present invention, the method of producing theferroelectric thin film or high dielectric thin film is characterized inthat the thin film is formed by a sputtering process, a Pulsed Laserdeposition process or a MOCVD (Metal Organic Chemical Vapor Deposition)process in an atmosphere of a mixed gas of oxygen and an inert gas. Thethin film may be formed by a spin coating process or a dip coatingprocess using a metal alkoxide or an organic acid salt as a startingmaterial in an atmosphere of a mixed gas of oxygen and an inert gasunder normal pressure.

In the method of producing the ferroelectric thin film or highdielectric thin film according to the present invention, the re-heatingtreatment is performed by a sputtering process, a Pulsed Laserdeposition process or a MOCVD (Metal Organic Chemical Vapor Deposition)process in ECR-oxygen plasma. The re-heating treatment may be performedby a spin coating process or a dip coating process using a metalalkoxide or an organic acid salt as a starting material by irradiationwith light in an ultraviolet region.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing a change in crystal structure of aferroelectric thin film depending on a concentration of oxygen in anatmosphere according to the present invention;

FIG. 2 is a sectional view showing a ferroelectric element according tothe present invention;

FIG. 3 is a sectional view showing a high dielectric element accordingto the present invention;

FIG. 4 is a sectional view showing a ferroelectric element in which aconductive oxide of the present invention is used for an electrode;

FIG. 5 is a schematic view showing a microstructure of the ferroelectricthin film of the present invention;

FIG. 6 is a sectional view showing a ferroelectric memory according tothe present invention;

FIG. 7 is a sectional view showing a high dielectric memory according tothe present invention;

FIG. 8 is a graph showing a degree of orientation of the (105) plane ofthe crystal structure depending on the concentration of oxygen in anatmosphere according to the present invention;

FIG. 9 is a graph showing a relationship between a voltage and a leakagecurrent density according to the present invention;

FIGS. 10(a) and 10(b) are views showing a non-contact type semiconductordevice using the ferroelectric element of the present invention; and

FIG. 11 is a graph showing the result of measuring a number of cycles ofoperation of the ferroelectric element of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described, withoutlimitation, by way of various. embodiments and with reference to thedrawings.

Principal reference numerals used in the drawings designate elements asfollows:

21, 31, 61, 71 and 1008 denote a top electrode; 41 denote a topelectrode (conductive oxide); 22, 42, 62 and 1007 denote a ferroelectricthin film; 32 and 72 denote a high dielectric thin film; 23, 33, 63, 73and 1006 denote a bottom electrode; 43 denotes a bottom electrode(conductive oxide); 24, 34 and 44 denote an underlying substrate; 25 and45 denote a ferroelectric element; 35 denotes a high dielectric element;64, 74 and 1002 denote an Si substrate; 65 and 75 denote a sourceregion; 66 and 76 denote a drain region; 67 and 79 denote polycrystalSi; 68, 77, and 78 denote SiO₂; 1001 denotes a non-contact typesemiconductor device; 1003 denotes a diffusion layer; 1004 denotes anSiO₂ gate layer; 1005 denotes a gate electrode; 1009 and 1010 denote anSiO₂ insulating layer; and 1011 denotes an aluminum interconnection.

(First Embodiment)

One embodiment of the present invention will be described.

A feature of the present invention will be described in detail. Theratio of occupation of a perovskite structure in the entire crystalphase of the thin film can be increased by controlling the atmospherefor forming a ferroelectric thin film or a high dielectric thin filmaccording to the present invention by reducing the concentration ofoxygen contained in the atmosphere. For a ferroelectric substanceSrBi₂Ta₂O₉, when the concentration of oxygen in the atmosphere isminimized, formation of a liquid phase caused by decomposition of oxidesis promoted even at a low temperature, to thereby achievecrystallization of the ferroelectric substance at a temperature lowerthan conventional, that is, 800° C. The reduction in the formationtemperature is also effective to prevent a reaction between theferroelectric substance and each of the top and bottom electrodes.

The ferroelectric thin film used in this embodiment is expressed by thechemical structural formula: (AO)²⁺(BCO)²⁻, where A is Bi, B is Sr, andC is Ta. Hereinafter, there will be described a method of producing sucha ferroelectric thin film. FIG. 2 shows the structure of a ferroelectricelement according to this embodiment, in which a bottom electrode 23 isformed on an underlying substrate, the ferroelectric thin film 22 isformed on the bottom electrode, and an top electrode 21 is formed on theferroelectric thin film. In addition, reference numeral 24 designatesthe underlying substrate. An Si base, on which an SiO₂ film was formedby thermal oxidation, was used as the underlying substrate 24. Thebottom electrode 23 (Pt) having a thickness of 2,000 Å was formed on theunderlying substrate 24 at room temperature by sputtering. To form theferroelectric dielectric thin film 22 on the bottom electrode 23, thesurplane of the bottom electrode 23 was spin-coated with a metalalkoxide solution having a composition of Bi:Sr:Ta=2:1:2 at 3,000 rpm(the number of rotation per minute) for 30 sec. The resultant substratewas dried at 150° C. for 10 min, and then, pre-heated in air or oxygenat a temperature lower than the crystallization temperature of theferroelectric thin film, concretely, at 500° C. for 15 min. The aboveprocedure was repeated three times to form a precursor thin film havinga thickness of 2,400 Å. Finally, the precursor thin film washeat-treated in an atmosphere containing oxygen at a varyingconcentration at 650° C. for 1 hr to produce a ferroelectric thin film.The crystal structure of the ferroelectric thin film was identified byX-ray diffraction.

FIG. 1 shows a change in the ratio of occupation of a perovskitestructure in the entire crystal phase of a thin film depending on theconcentration of oxygen in atmospheric gas. The reduced concentration ofoxygen functions to increase the ratio of occupation of the perovskitestructure. More specifically, the increase in the ratio of occupation ofthe perovskite structure is maximized at the concentration of oxygen ina range of 0.2 to 3.0%. As a result, the concentration of oxygen in theatmospheric gas for forming the ferroelectric thin film may bepreferably in a range of more than 0.1% and less than 5.0%. When theconcentration of oxygen is equal to or less than 0.1%, the amount ofoxygen required for forming the perovskite structure is insufficient, sothat it is difficult to form the perovskite structure. When it is equalto or more than 5.0%, the effect of forming the perovskite structure isno longer obtained. FIG. 1 also shows a change in the ratio ofoccupation of the perovskite structure in the entire crystal phase ofthe thin film depending on the concentration of oxygen in the atmospherewhen the formation temperature is changed in a range of 600 to 700° C.The effect of the low oxygen concentration exerted on the ratio ofoccupation of the perovskite structure becomes more effective as theformation temperature becomes lower. According to this embodiment, itmay be desirable to form a ferroelectric thin film at a temperature of650° C. or less and to form a high dielectric thin film at a temperatureof 600° C. or less. In this case, the bottom limit of the formationtemperature may desirably be set at 400° C., because it is difficult toform the perovskite structure by heat treatment at a temperature lessthan 400° C.

FIG. 8 shows a relationship between a degree of orientation of the (105)plane of the perovskite structure and a concentration of oxygen. Thedegree of orientation is expressed by a ratio of a peak intensity I(105)of the (105) plane to a peak intensity I(total) of the total crystalplanes identified by X-ray diffraction. When the concentration of oxygenis less than 5%, the degree of orientation of the (105) plane becomeslarge. This means that the crystal structure of the SrBi₂Ta₂O₉ferroelectric thin film formed at a low oxygen concentration is stronglyoriented at the (105) plane. The reason for this is that a liquid phaseis partially produced by decomposition of oxides of the components at alow oxygen concentration and crystal growth is originated from theliquid phase, to thereby make it easy to achieve preferential growth ofthe (105) plane. That is to say, the low oxygen concentration iseffective to make it easy to set the orientation of the crystalstructure at the (105) plane. The SrBi₂Ta₂O₉ ferroelectric thin filmhaving a layered perovskite structure exhibits a crystal anisotropyallowing the polarization axis to be directed only in parallel to theBi—O layer (perpendicular to the c-axis) because of the symmetry of thecrystals. However, since the SrBi₂Ta₂O₉ ferroelectric thin film in thisembodiment is preferentially oriented at the (105) plane, such a thinfilm can exhibit good characteristics. In addition, even a differentferroelectric material can be preferentially oriented at a crystal planeallowing the polarization axis to be directed in the vertical direction.

Next, a Pt film having a thickness of 2,000 Å was formed on theferroelectric thin film 22 expressed by the chemi-cal-structuralformula, (BiO)²⁺(SrTaO)²⁻, at room temperature by sputtering, to formthe top electrode 21 on the ferroelectric thin film 22. In this way, aferroelectric element 25 was obtained. The spontaneous polarization (Pr)and the coercive field (Ec) of the ferroelectric element thus obtainedwere measured at room temperature. The results are shown in Table 1.

TABLE 1 Conc. of O₂ (%) 0.15 0.2 0.7 1.0 3.0 5.0 Pr (μC/cm²)  7 17 20 1817  6 Ec (kV/cm) 70 52 45 50 53 74 Number of re- 1E+14 1E+14 1E+14 1E+141E+14 1E+13 writing operations

The value Pr is a polarization quantity obtained at a positive ornegative maximum applied voltage in the hysteresis of the Pr-V curve.The value Pr is high and the value Ec is low at a concentration ofoxygen in a range of 0.2 to 3.0%. This corresponds to the results ofX-ray diffraction. In particular, the ferroelectric thin film formed atan oxygen concentration of 0.7% exhibits a value Pr of 20 μC/cm² and avalue Ec of 45 kV/cm. Each ferroelectric thin film formed in thisembodiment was measured in terms of the number of cycles in which avoltage of 136 kV/cm was repeatedly applied to the thin film with thepolarity of the voltage reversed. FIG. 11 typically shows themeasurement result of the ferroelectric thin film formed at the oxygenconcentration of 0.7%. For each ferroelectric thin film formed at anoxygen concentration in a range of 0.15 to 3.0%, degradation of the Prcharacteristic is not observed until the number of cycles reaches 10¹⁴times.

A ferroelectric element including a ferroelectric thin film having achemical structural formula of (AO)²⁺(Sr,TaO)²⁻ where the element of theA site is selected from a group consisting of Tl, Hgl Pb, Sb and As wasproduced in the same procedure as that described above. The values Prand Ec of each ferroelectric element thus obtained were measured. As aresult, the value Pr was in a range of 19 to 21 μC/cm² and the value Ecwas in a range of 44 to 48 kV/cm.

A ferroelectric element including a ferroelectric thin film expressed bythe chemical structural formula: (BiO)²⁺(BTaO)²⁻, where the element ofthe B site is selected from a group consisting of Pb, Ca and Ba wasproduced using the same procedure as that described above. The values Prand Ec of the ferroelectric element thus obtained were measured. As aresult, the value Pr was in a range of 18 to 22 μC/cm² and the value Ecwas in a range of 43 to 47 kV/cm.

A ferroelectric element including a ferroelectric thin film expressed bythe chemical structural formula: (BiO)²⁺(SrCO)²⁻, where the element ofthe C site is selected from a group consisting of Ti, Nb, W, Mo, Fe, Coand Cr was produced using the same procedure as that described above.The values Pr and Ec of the ferroelectric element thus obtained weremeasured. As a result, the value Pr was in a range of 17 to 22 μC/cm²and the value Ec was in a range of 42 to 49 kV/cm.

Moreover, in the first embodiment, since the ferroelectric thin film canbe formed at a low temperature by reducing the concentration of oxygen,there is no problem in the formation of a transient layer or withdiffusion of the elements, so that a structure may be adopted in whichno diffusion preventive layer is provided between the thin film and theunderlying substrate.

(Second Embodiment)

The ferroelectric thin film used in this embodiment is expressed by achemical structural formula: (Pb/A) (Zr/Ti)O₃, where A is La.Hereinafter, there will be described a method of producing such aferroelectric thin film. In FIG. 2, which is a sectional view of theferroelectric element, reference numeral 24 designates an underlyingsubstrate. An Si base, on which an SiO₂ film was formed by thermaloxidation, was used as the underlying substrate 24. A Pt film having athickness of 2,500 Å, as a bottom electrode 23, was formed on theunderlying substrate 24 in vacuum at room temperature by sputtering. Toform a ferroelectric dielectric thin film 22 on the bottom electrode 23,the surplane of the bottom electrode 23 was spin-coated with a metalalkoxide solution having a composition ofPb:La:Zr:Ti=0.95:0.05:0.52:0.48 at 2,500 rpm for 30 sec. The resultantsubstrate was dried at 140° C. for 13 min, and then, pre-heated in airor oxygen at a temperature lower than the crystallization temperature ofthe ferroelectric thin film 22, concretely, at 450° C. for 20 min. Theabove procedure was repeated three times to form a precursor thin filmhaving a thickness of 1,700 Å. Thereafter, the precursor thin film washeat-treated in a low oxygen atmosphere at 550° C. to produce theferroelectric thin film 22 expressed by the chemical structural formula:(Pb/A) (Zr/Ti)O₃. The crystal structure of the ferroelectric thin filmwas identified by X-ray diffraction. The results showed that, like thefirst embodiment, the ratio of occupation of the perovskite structure inthe entire crystal phase of the thin film was rapidly increased at theconcentration of oxygen in a range of 0.2 to 3.0%. Next, a Pt filmhaving a thickness of 2,000 Å, as an top electrode 21, was formed on theferroelectric thin film 22 expressed by the chemical structural formula,(Pb/A) (Zr/Ti)O₃, in a vacuum at room temperature by sputtering. In thisway, a ferroelectric element 25 was obtained. The spontaneouspolarization (Pr) and the coercive field (Ec) of the ferroelectricelement thus obtained were measured. As a result, the value Pr was 20μC/cm² and the value Ec was 50 kV/cm at the oxygen concentration of0.7%. Further, the dielectric constant (∈) of the ferroelectric elementwas measured at room temperature. The results are shown in Table 2.

TABLE 2 Conc. of O₂ (%) 0.15 0.2 0.7 1.0 3.0 5.0 ε 1320 1560 1590 15701564 1290 J(A/cm²) at 3V 7E−7 2E−7 1E−7 3E−7 4E−7 3E−6

The value ∈ was 1590 at the oxygen concentration of 0.7%. Further, theferroelectric element was measured in terms of a relationship between anapplied voltage and a leakage current density. As a result, the leakagecurrent density was 1×10⁻⁷ A/cm² or less at an applied voltage of 3 V.Accordingly, it was confirmed that the ferroelectric element in thisembodiment had a very good withstand voltage characteristic.

A ferroelectric element including a ferroelectric thin film expressed bythe chemical structural formula: (Pb/A) (Zr/Ti)O₃, where A is Ba or Nb,was produced using the same procedure as that described above. Thevalues Pr and Ec of each ferroelectric element were measured. As aresult, the value Pr was 20 μC/cm² and the value Ec was 51 kV/cm. Thedielectric constant of the ferroelectric element was also estimated atroom temperature. As a result, the ferroelectric element exhibited adielectric constant which was as high as 1590 to 1610 at an oxygenconcentration of 0.2 to 3.0%. In particular, even the ferroelectric thinfilm expressed by the chemical structural formula: (Pb/A) (Zr/Ti)O₃,where A is Ba, Nb or Ti, which was orientated at the (111) plane,exhibited a high polarization characteristic.

(Third Embodiment)

A method of producing a high dielectric thin film having a compositionof (Ba_(0.5)Sr_(0.5))TiO₃ according to this embodiment will bedescribed. In FIG. 3, which is a sectional view of a high dielectricelement, reference numeral 34 designates an underlying substrate. Thesame Si base as that in the second embodiment was used as the underlyingsubstrate 34. A Pt film having a thickness of 2,000 Å, as a bottomelectrode 33, was formed on the underlying substrate 34 in a vacuum atroom temperature by sputtering. To form a ferroelectric dielectric thinfilm 32 on the bottom electrode 33, a precursor thin film having athickness of 100 nm was formed on the bottom electrode 33 in a mixed gasof oxygen and argon at a temperature of 300° C. and at a pressure of0.55 Pa. To form a perovskite structure, the precursor thin film washeat-treated in the atmosphere containing oxygen at a low concentrationat 500° C., to form a high dielectric thin film having the compositionof (Ba_(0.5)Sr_(0.5))TiO₃. The crystal structure of the high dielectricthin film was examined by X-ray diffraction. The results showed that,like the first embodiment, the ratio of occupation of the perovskitestructure in the entire crystal phase of the thin film began to beincreased as the oxygen concentration was reduced to less than 5%, andit was maximized at an oxygen concentration of 0.2 to 3.0%. Then, a Ptfilm having a thickness of 2,000 Å, as an top electrode 31, was formedon the high dielectric thin film 32 having the composition of(Ba_(0.5)Sr_(0.5))TiO₃ in a vacuum at room temperature by sputtering. Inthis way, a high dielectric element 35 was obtained. The dielectricconstant (∈) of the high dielectric element 35 was measured at roomtemperature. The results are shown in Table 3.

TABLE 3 Conc. of O₂ (%) 0.15 0.2 0.7 1.0 3.0 5.0 ε 310 493 520 503 480253

The high dielectric element exhibited a dielectric constant which was ashigh as 480 to 520 at an oxygen concentration of 0.2 to 3.0%.

(Fourth Embodiment)

The ferroelectric thin film used in this embodiment is expressed by thechemical structural formula: (AO)²⁺(BCO)²⁻, where A is Bi, B is Sr, andC is Nb. Hereinafter, there will be described a method of producing sucha ferroelectric thin film. In FIG. 4, which is a sectional view of aferroelectric element, reference numeral 44 designates an underlyingsubstrate. An Si base, on which an SiO₂ film was formed by thermaloxidation, was used as the underlying substrate 44. A film (thickness:1,700 Å) made from conductive oxide of a single element, RuO, was formedon the underlying substrate 44 in an oxygen gas atmosphere at 450° C. bysputtering. To form a ferroelectric thin film on the bottom electrode43, the surplane of the bottom electrode 43 was spin-coated with a metalalkoxide solution containing Bi, Sr and Nb at 3,000 rpm for 25 sec. Theresultant substrate was dried at 150° C. for 10 min, and then,pre-heated in air or oxygen at 450° C. for 10 min. the above procedureas repeated three times to form a precursor thin film having a thicknessof 2,300 A. The precursor thin film was heated in a low oxygenconcentration atmosphere containing argon gas and oxygen at aconcentration of 0.7% at 600° C. to produce a ferroelectric thin film42, (BiO)²⁺(SrNbo)²⁻, having the perovskite structure. Next, a film(thickness: 1,700 Å) made from the conductive oxide of the singleelement RuO, as a top electrode 41, was formed on the ferroelectric thinfilm 42 in an oxygen gas atmosphere at 450° C. by sputtering. In thisway, a ferroelectric element 45 was produced. The spontaneouspolarization (Pr) and the coercive field (Ec) of the ferroelectricelement 4S thus obtained were measured at room temperature. As a result,the value Pr was 19 μPC/cm² and the value Ec was 46 kV/cm.

Each ferroelectric element having the same structure as that describedabove except that the electrode material was changed from RuO into anyone of TiO_(x), VO_(x), EuO, CrO₂, MoO₂, WO₂, PhO, OsO, IrO, PtO, ReO₂,RuO₂ and SnO₂, was produced using the same procedure as that describedabove. The values Pr and Ec of each ferroelectric element were measured.As a result, the value Pr was in a range of 18 to 22 μC/cm² and thevalue Ec was in a range of 44 to 48 kV/cm. As described above, by use ofa conductive oxide of a single element, which has a resistivity of 1 mΩ·cm or less for ensuring a function as a metal or an electrodematerial, or one kind of conductive oxide having the perovskitestructure, as the material of each of the top and bottom electrodes usedin this embodiment, there can be produced an oxide dielectric elementhaving good electrical characteristics.

(Fifth Embodiment)

A bottom electrode (Pt) was formed on an underlying substrate inaccordance with the same production method as that in the firstembodiment. Next, to form a ferroelectric thin film expressed by thechemical structural formula: (AO)²⁺(BCO)²⁻, where A is Bi, B is Sr, andC is Ta, the bottom electrode was spin-coated with a metal alkoxidesolution having the same composition as that in the first embodiment at3,000 rpm for 35 sec. The resultant substrate was dried at 150° C. for10 min, and then, pre-heated in air or oxygen at 400° C. for 10 min. Theabove procedure was repeated twice to form a precursor thin film havinga thickness of 1,100 Å. The precursor thin film was heated in anatmosphere containing oxygen at a low concentration at 630° C. toproduce a ferroelectric thin film. For comparison, a ferroelectric thinfilm, which was prepared by forming a bottom electrode on an underlyingsubstrate and forming a ferroelectric thin film having the samecomposition as that described above at a low oxygen concentration inaccordance with the same procedure as that described above, was furtherheated in an ECR oxygen plasma at 400° C. An top electrode (Pt) wasformed on each ferroelectric thin film thus obtained, using the sameprocedure as that in the first embodiment, to thus produce aferroelectric element having the cross-sectional structure shown in FIG.2. The spontaneous polarization (Pr) and the coercive field (Ec) of eachferroelectric element were measured at room temperature. The results areshown in Table 4.

TABLE 4 Atmosphere No heating Radical oxygen N₂O O₃ Pr 19 30 27 28(μC/cm²) Ec (kV/cm) 43 35 34 31

The ferroelectric element re-heated in the ECR oxygen plasma is higherin spontaneous polarization and lower in coercive field than aferroelectric element not subjected to reheating treatment. Similarly,the ferroelectric element was re-heated using each of O₃, radicaloxygen, and N₂O (nitrous oxide). Each of the ferroelectric elements thusre-heated exhibited a spontaneous polarization and a coercive fieldcomparable to those of the ferroelectric element re-heated in the ECRoxygen plasma. As described above, the re-heating treatment of aferroelectric thin film in an activated oxygen atmosphere having astrong oxidizing function forms the perovskite structure without anyloss in oxygen, to thereby remarkably enhance the electricalcharacteristics of the ferroelectric element, including theferroelectric thin film. The re-heating treatment may be preferablyperformed at a temperature equal to or less than the crystallizationtemperature of a ferroelectric thin film at a low oxygen concentration.

(Sixth Embodiment)

A bottom electrode (Pt) was formed on an underlying substrate inaccordance with the same production method as that used in the firstembodiment. Next, to form a ferroelectric thin film expressed by thechemical structural formula: (AO)²⁺(BCO)²⁻, where A is Bi, B is Sr, andC is Ta, the bottom electrode was spin-coated with a metal alkoxidesolution having a composition of Bi:Sr:Ta=2.2:1:2 at 3,500 rpm for 25sec. The resultant substrate was dried at 170° C. for 10 min, and then,preheated at 450° C. for 10 min. The above procedure was repeated threetimes to form a precursor thin film having a thickness of 2,200 Å. Theprecursor thin film was heat-treated in an atmosphere containing oxygenat a concentration of 0.7% at 650° C. for 1 hr to form a ferroelectricthin film. For comparison, a precursor thin film prepared in accordancewith the same procedure as that described above was heat-treated in anatmosphere of 100% oxygen at 650° C. for each treatment time of 1 hr and5 hr, to form a ferroelectric thin film. A top electrode (Pt) was formedon each of the ferroelectric elements thus obtained using the sameprocedure as that used in the first embodiment, to thus produce aferroelectric element having the cross-sectional structure shown in FIG.2. The Pr of each ferroelectric element was measured at roomtemperature. As a result, for the ferroelectric element formed at theoxygen concentration of 0.7%, the value Pr μC/cm² was 22 while for theferroelectric element (treatment time: 1 hr) formed at the oxygenconcentration of 100%, the hysteresis curve of polarization was notobserved and for the ferroelectric element (treatment time: 0.5 hr)formed at the oxygen concentration of 100%, the value Pr was as low as10 μC/cm². In this way, it is seen that a decrease in the oxygenconcentration has an effect of shortening the heat treatment time. Thereason for this is that, as described above, the rate of crystal growthwhich originates from a liquid phase caused by decomposition of oxidesin the components is increased by a decrease in oxygen concentration,and accordingly, the thin film formed at a low oxygen concentrationforms the perovskite structure in a short treatment time which is aboutone-fifth that of the thin film formed at the conventional oxygenconcentration (100%), to thereby obtain high electrical characteristics.The results of the component analysis for each ferroelectric thin filmshow that the ferroelectric thin film formed in the atmosphere of a lowoxygen concentration has a stoiehiometric composition of Sr:Bi:Ta=1:2:2;while the ferroelectric thin film formed in the atmosphere of 100%oxygen has a Bi-rich composition of Sr:Bi:Ta=1:2.2:2. In thisembodiment, a ferroelectric thin film is formed in an atmosphere of alow oxygen concentration at a low temperature, and accordingly, forexample, a SrBi₂Ta₂O₉ ferroelectric thin film having a stoichiometriccomposition can be formed irrespective of the starting content of Bi.This eliminates the necessity of making the starting content of Biexcessive. Even if the starting content of Bi is excessive, it ispossible to suppress the formation of an irregular phase containing Biin a large amount at grain boundaries of a ferroelectric layer afterformation of a ferroelectric thin film, and hence to enhance thewithstand voltage characteristic of the thinfilm. Further, since thereis no reaction between the ferroelectric thin film and each of the topand bottom electrodes,, it is possible to enhance the dielectricconstant of the ferroelectric thin film.

In this embodiment, description is made by way of example concerning theuse of a SrBi₂Ta₂O₉ ferroelectric thin film; however, even for an oxidedielectric thin film expressed by the chemical structural formula:Pb(Zr/Ti)O₃, (Ba/Sr)TiO₃ or the like, the heat treatment time requiredfor forming the thin film can be shortened.

(Seventh Embodiment)

A Pt bottom electrode having a thickness of 2,000 Å was formed on anunderlying substrate composed of an Si base on which SiO₂ was formed, inaccordance with the same production method as that used in the firstembodiment. To form a ferroelectric thin film on the Pt bottomelectrode, the bottom electrode was spin-coated with a metal alkoxidesolution having a composition of Bi:Sr:Ta=2:1:2 at 2,000 rpm for 30 sec.The resultant substrate was dried at 150° C. for 15 min, and then,pre-heated at 450° C. for 20 min. The above procedure was repeated fivetimes to form a precursor thin film having a thickness of 2,000 Å. Theprecursor thin film was heat-treated in an atmosphere containing oxygenat a concentration of 0.7% at 650° C. for 1 hr to produce aferroelectric thin film. For comparison, the same precursor thin filmwas heat-treated at each of 800° C. and 720° C. for 1 hr in anatmosphere of 100% oxygen, to form a ferroelectric thin film. A Pt topelectrode having a thickness of 2,000 Å was formed on the surplane ofeach ferroelectric thin film by sputtering, to thus produce aferroelectric element. The withstand voltage characteristic of eachferroelectric element thus obtained was measured. The results are shownin FIG. 9. The ferroelectric element formed at 650° C. in the atmosphereof an oxygen concentration of 0.7% exhibits a leakage current density of3.0×10⁻⁹ A/cm² even at a voltage of 5 V, and therefore, it is superiorin withstand voltage characteristic to the ferroelectric element formedat the conventional heat treatment temperature of 800° C. or 720° C.

FIG. 5 is a schematic view of a micro-structure of the SrBi₂Ta₂O₉ferroelectric substance obtained in this embodiment. From themicro-structure shown in FIG. 5, it becomes apparent that crystal grainsof the ferroelectric thin film obtained in the atmosphere of a lowoxygen concentration at a low temperature have an average grain size ofabout 70 nm or less, that is, the crystal grains are smaller and denserthan those of the ferroelectric thin film obtained at a hightemperature. Accordingly, the ferroelectric thin film obtained in thisembodiment is small in leakage current density and good in withstandvoltage characteristic.

Further, as a result of measuring the withstand voltage of the(Ba_(05.)Sr_(0.5))TiO₃ high dielectric thin film formed in the thirdembodiment, the leakage current density was 5.0×10⁻⁷ A/cm². Accordingly,it was confirmed that the above ferroelectric thin film had a goodwithstand voltage characteristic.

(Eighth Embodiment)

FIG. 6 is a sectional view of a ferroelectric memory using aferroelectric element according to this embodiment. The ferroelectricmemory has a structure including a MOS-transistor in which an oxidelayer, a metal layer and an insulating layer are formed on asemiconductor field effect transistor structure, and a capacitorcomposed of the above ferroelectric element shown in FIG. 2. Theproduction method will be described below. First, the surplane of an Sisubstrate 64 having a source region 65 and a drain region 66 wasoxidized to form an SiO₂ film having a thickness of 260 Å. The SiO₂ filmwas mask-patterned to form an SiO² film 68 at a central portion of thesubstrate. A polycrystal Si film 67 having a thickness of 4,500 Å wasformed on the projecting Si film 68 by CVD. The ferroelectric elementhaving a structure including the top electrode 61, ferroelectric thinfilm, and low electrode 63 produced in the first embodiment was formedon the polycrystal Si film 67. In this way, a ferroelectric memory usingthe ferroelectric element was obtained. This ferroelectric memory isadvantageous in that a difference in capacitance caused by reversal ofan electric field can be detected at double the magnitude.

(Ninth Embodiment)

FIG. 7 is a sectional view showing a high dielectric memory using a highdielectric element according to this embodiment. The production methodwill be described below. First, the surplane of an Si substrate 74having a source region 75 and a drain region 76 was oxidized to form anSiO₂ film having a thickness of 270 Å. The SiO₂ film was mask-patternedto form a projecting SiO₂ film 78 at a central portion of the substrate.A polycrystal Si film 79 having a thickness of 4,600 Å was formed on theprojecting Si film 78 by CVD, and then, the surplane of the Si substrate74 was oxidized to form an SiO₂ film 77 having a thickness of 250 Å,thus producing a MOS transistor portion. The high dielectric elementhaving the structure including the top electrode 71, high dielectricthin film 72 and bottom electrode 73 produced in the third embodimentwas formed on a capacitor portion opposed to the semiconductor MOSportion thus obtained. In this way, a high dielectric memory using thehigh dielectric element was obtained. The high dielectric memory allowsdetection by a change in stored charge capacity obtained by a voltage of3 V.

(Tenth Embodiment)

FIG. 10(a) shows a non-contact type semiconductor device 1001, and FIG.10(b) shows the structure of a ferroelectric element A contained in thenon-contact type semiconductor device. The ferroelectric element wasformed as follows: namely, an SiO₂ gate film 1004 was formed on an Sisubstrate 1002 having a diffusion layer 1003, and was mask-patterned toform a gate electrode 1005. A ferroelectric capacitor includes a Ptbottom electrode 1006, an (SrBi₂Ta₂O₉) ferroelectric thin film 1007formed at a low oxygen concentration, and a Pt top electrode 1008. Toseparate the transistor from the capacitor, SiO2 insulating layers 1009and 1010 were formed, and the top electrode 1008 was connected to thediffusion layer 1003 via an aluminum interconnection 1011. A systemusing a non-contact semiconductor device includes a controller, aresponsor containing a memory and a communication device, and an IC cardcontaining the non-contact type semiconductor device, wherein a signalfrom the controller is transmitted to the IC card, and informationnecessary for the IC card is fed back to the controller on the basis ofa command. The use of a non-volatile RAM for a memory element allows thereversal time of the ferroelectric substance to be one nanosecond orshorter. As a result, the system has various good performances in thatan information reading operation is performed at an equidistance with aninformation writing operation, the transmission speed of data is high,and the rate of occurrence of an error upon writing is extremelyreduced.

In the above embodiment, by way of example, a ferroelectric elementhaving the Pt top electrode 1008, SrBi₂Ta₂O₉ ferroelectric thin film1007, and bottom electrode 1006 has been described; however, a highdielectric element having an top electrode, a high dielectric thin film,and a bottom electrode may be provided. A semiconductor device using ahigh dielectric element has a 30 fF stored charge capacity at a voltageof 3 V.

As described above, the use of the ferroelectric element in thisembodiment can provide a non-contact type semiconductor having goodelectrical characteristics.

As described above, since the liquid phase due to decomposition ofoxides in the components is produced and the rate of crystal growthwhich originates from the liquid phase is increased when the oxideferroelectric thin film and the oxide high dielectric thin film areformed in the atmosphere of the reduced concentration of oxygen, it ispossible to form the oxide ferroelectric thin film and the oxide highdielectric thin film at temperatures lower than conventional,particularly, 650° C. or less and 600° C. or less, respectively, andfurther, to shorten the heat treatment time required for forming thethin film. As a result, in the thin film formed according to the presentinvention, the crystal structure is preferentially oriented at thecrystal plane allowing the polarization axis to be directed in thevertical direction; the average crystal grain size is controlled at anoptimum value; and reaction with the electrode is prevented. This makesit possible to form an oxide dielectric element having a high dielectricconstant, the high spontaneous polarization, and a small coercive field.A ferroelectric memory for detecting reading and writing operations canbe produced by incorporating the above ferroelectric element in thesemiconductor field effect transistor, and a high dielectric memory fordetecting reading and writing operations can be also produced byincorporating the above high dielectric element in the semiconductor MOSstructure. Further, a semiconductor device using the ferroelectricmemory or high dielectric memory as the non-contact type reading orwriting memory can be produced.

In this way, the present invention is effective for application to ahighly integrated ferroelectric element and high dielectric element, anda semiconductor device using such an element.

What is claimed is:
 1. A method of producing an oxide dielectric elementincluding a top electrode, an oxide dielectric thin film, and a bottomelectrode, comprising the steps of: forming said oxide dielectric thinfilm in an atmosphere containing oxygen at a concentration of more than0.1% and less than 5% and at a temperature of 650° C. or less; andreheating said oxide dielectric thin film in an activated oxygenatmosphere.
 2. A method of producing an oxide dielectric elementincluding a top electrode, an oxide dielectric thin film, and a bottomelectrode, comprising the steps of: forming said oxide dielectric thinfilm in an atmosphere containing oxygen at a concentration of more than0.1% and less than 5% and at a temperature of 600° C. or less; andreheating said oxide dielectric thin film in an activated oxygenatmosphere.
 3. A method of producing an oxide dielectric elementaccording to claim 1 or claim 2, wherein said oxide dielectric thin filmis expressed by a composition of (Ba/Sr)TiO₃.
 4. A method of producingan oxide dielectric element according to claim 1 or claim 2, whereinsaid oxide dielectric thin film is formed by a sputtering process, aPulsed Laser deposition process or a MOCVD (Metal Organic Chemical VaporDeposition) process, which is performed in an atmosphere of mixed gas ofoxygen and inert gas.
 5. A method of producing an oxide dielectricelement according to claim 1 or claim 2, wherein said oxide dielectricthin film is formed by a spin-coating process or a dip-coating processusing metal alkoxide or organic acid salt as a starting material, saidprocess being performed in an atmosphere of a mixed gas of oxygen and aninert gas at atmospheric pressure.
 6. A method of producing an oxidedielectric element according to claim 1 or claim 2, wherein said oxidedielectric thin film is re-heated by a sputtering process, a laservapor-deposition process or a MOCVD process, which is performed in anECR oxygen plasma.
 7. A method of producing an oxide dielectric elementaccording to claim 1 or claim 2, wherein the re-heating treatment isperformed by a spin-coating process or a dip-coating process using metalalkoxide or organic acid salt as a starting material by irradiation withlight in an ultraviolet region.
 8. A method of producing an oxidedielectric element including a top electrode, an oxide dielectric thinfilm, and a bottom electrode, wherein: said oxide dielectric thin filmhas a composition expressed by a chemical structural formula(Pb/A)(Zr/Ti)O₃, where A is one element selected from a group consistingof La, Ba and Nb; and the leakage current density of said oxidedielectric thin film is 10⁻⁶ A/cm² or less at a voltage of 5 V or less,and wherein the method includes the step of forming the oxide dielectricthin film at a temperature of 550° C. or less.
 9. A method of producingan oxide dielectric thin film including a top electrode, an oxidedielectric thin film, and a bottom electrode, characterized in that saidoxide dielectric thin film is expressed by a chemical structuralformula: (AO)²⁺(BCO)²⁻, where A is one element selected from a groupconsisting of Bi, Ti, Hg, Pb, Sb and As; B is at least one elementselected from a group consisting of Pb, Ca, Sr, Ba and rare earthelements; and C is at least one element selected from a group consistingof Ti, Nb, Ta, W, Mo, Fe, Co and Cr; and comprising the step of: formingsaid oxide dielectric thin film in an atmosphere containing oxygen at aconcentration of more than 0.1% and less than 5% and at a temperature of650° C. or less.
 10. A method of producing an oxide dielectric elementincluding a top electrode, an oxide dielectric thin film, and a bottomelectrode, comprising the step of: forming said oxide dielectric thinfilm in an atmosphere containing oxygen at a concentration of more than0.1% and less than 5% and at a temperature of 650° C. or less, whereinsaid oxide dielectric thin film is formed by a sputtering process, aPulsed Laser deposition process or a MOCVD (Metal Organic Chemical VaporDeposition) process, which is performed in an atmosphere of mixed gas ofoxygen and inert gas.
 11. A method of producing an oxide dielectricelement including a top electrode, an oxide dielectric thin film, and abottom electrode, comprising the step of: forming said oxide dielectricthin film in an atmosphere containing oxygen at a concentration of morethan 0.1% and less than 5% and at a temperature of 600° C. or less,wherein said oxide dielectric thin film is formed by a sputteringprocess, a Pulsed Laser deposition process or a MOCVD (Metal OrganicChemical Vapor Deposition) process, which is performed in an atmosphereof mixed gas of oxygen and inert gas.